We made them 30x30 with 3/4 plywood, 2x6 on edge in the back, 2x2 on the front (both underneath, top should be smooth). Make a little triangle support on each side for the 2x6 or it will wobble.
Cut a hole 1/4" larger than the washer all the way around (1/2" diameter larger). Covered in carpet and glued a capped piece of PVC underneath to catch the washers.
Put an eye in the front of each board and cut light chain with clips in a length that makes the center of the holes 21' apart when pulled tight.
I built them in 2003, they are still going strong (re-covered once)
Cody
Fightin Texas Aggie c/o 04
[This message has been edited by dahouse (edited 8/8/2012 10:15a).]